The invention relates to elements faced with superhard material, and particularly to preform elements comprising a facing table of superhard material having a front face, a peripheral surface, and a rear surface bonded to a substrate of material which is less hard than the superhard material.
Preform elements of this kind are often used as cutting elements on rotary drag-type drill bits, and the present invention will be particularly described in relation to such use. However, the invention is not restricted to cutting elements for this particular use, and may relate to preform elements for other purposes. For example, elements faced with superhard material, of the kind referred to, may also be employed in workpiece-shaping tools, high pressure nozzles, wire-drawing dies, bearings and other parts subject to sliding wear, as well as elements subject to percussive loads as may be the case in tappets, cams, cam followers, and similar devices in which a surface of high wear resistance is required.
Preform elements used as cutting elements in rotary drill bits usually have a facing table of polycrystalline diamond, although other superhard materials are available, such as cubic boron nitride. The substrate of less hard material is often formed from cemented tungsten carbide, and the facing table and substrate are bonded together during formation of the element in a high pressure, high temperature forming press. This forming process is well known and will not be described in detail.
Each preform cutting element may be mounted on a carrier in the form of a generally cylindrical stud or post received in a socket in the body of the drill bit. The carrier is often formed from cemented tungsten carbide, the surface of the substrate being brazed to a surface on the carrier, for example by a process known as "LS bonding". Alternatively, the substrate itself may be of sufficient thickness as to provide, in effect, a cylindrical stud which is sufficiently long to be directly received in a socket in the bit body, without being brazed to a carrier. The bit body itself may be machined from metal, usually steel, or may be moulded using a powder metallurgy process.
Such cutting elements are subjected to extremes of temperature during formation and mounting on the bit body, and are also subjected to high temperatures and heavy loads when the drill is in use down a borehole. It is found that as a result of such conditions spalling and delamination of the superhard facing table can occur, that is to say the separation and loss of the diamond or other superhard material over the cutting surface of the table.
This may also occur in preform elements used for other purposes, and particularly where the elements are subjected to repetitive percussive loads, as in tappets and cam mechanisms.
Commonly,in preform elements of the above type the interface between the superhard table and the substrate has usually been flat and planar. However, particularly in cutting elements for drill bits, attempts have been made to improve the bond between the superhard facing table and the substrate by configuring the rear face of the facing table so as to provide a degree of mechanical interlocking between the facing table and substrate.
One such arrangement is shown in U.S. Pat. No. 5,120,327 where the rear surface of the facing table is integrally formed with a plurality of identical spaced apart parallel ridges of constant depth. The facing table also includes a peripheral ring of greater thickness, the extremities of the parallel ridges intersecting the surrounding ring. U.S. Pat. No. 4,784,023 illustrates a similar arrangement but without the peripheral ring.
While such cutting elements have met with some success in the field, they suffer from certain serious disadvantages. Since the ridges on the facing table are parallel and extend from one side of the cutting element to the other, it is necessary; to ensure that the cutting element is mounted on the drill bit in the correct rotational orientation, since it is desirable that the cutting edge, i.e. the part of the periphery of the facing table which engages the formation during drilling, extends across the ends of the ridges. This leads to difficulties, during manufacture, in mounting such cutting elements in the correct orientation on the posts, since many posts are not mounted on the bit body so as to be perpendicular to the formation. Thus the required orientation of the cutting element on the post depends upon the ultimate orientation and alignment of the post when mounted on the bit body.
One of the main purposes of providing the above-mentioned ridges on the facing table is to improve the bonding between the facing table and the substrate by accommodating the distortion which results from heating of the cutting assembly during manufacture, both during formation of the cutting element itself, and in its subsequent bonding onto a carrier. Such distortion results from the difference in coefficient of thermal expansion between the superhard material of the facing table and the less hard material of the substrate. Since the cutting elements of the kind referred to above are not symmetrical about the central axis, the distortion as a result of heating is also not symmetrical. Thus little distortion may be found along the plane of the ridges, but considerable distortion may be evident perpendicular to the ridges. This can therefore lead to splitting along the line of the ridges when the cutting element is subjected to high temperatures, for example when bonding to a carrier.
Given the difference in properties between the superhard material and the material of the substrate, a stress condition is always established between the facing table and the substrate. This stress is generally radial and the intensity of the stress increases with distance from the centre of the cutter. The prior art cutting elements take no account of this and provide no arrangement for accommodating the increasing stress at the outer radial positions. The opposite extremities of the parallel ridges provide some reinforcement of the outer peripheral ring on two diametrically opposed portions of the peripheral ring but provide no such support for the opposed portions of the ring which lie at opposite ends of a diameter at right angles to the ridges.